26-inch tires are definitely slower than 700c over distance. I tend to go as fast commuting to work and such, but my speed is definitely slower on any ride that lasts more than an hour.
I ride with a standard and inexpensive 1.5-1.75 inch road-type tire with little or no tread. Usually Performance's house-brand, or right now Kenda. For flat protection, I ride with a 20 year-old slime barrier strip between the tire and the tube. It is clunky, and definitely slows me down, but I also have fewer than 3 flats per year. It is worth it.
Barrier strips don't work that well on 700c tires, but they really fit the bill with 26-inch tires. IMO.